prune support (clean up build cache)

[Constantin07]

Docker has a nice feature to remove old things like:

docker system prune -fa

Am I supposed to delete images individually with img?

# img rm -h
rm -  Remove one or more images.

Usage: img rm [OPTIONS] IMAGE [IMAGE...]

Flags:
  -h, --help   help for rm

Global Flags:
  -b, --backend string   backend for snapshots ([auto native overlayfs]) (default "auto")
  -d, --debug            enable debug logging
  -s, --state string     directory to hold the global state (default "/root/.local/share/img")

The wildcard doesn't work either.

Yeah, I know about:

img ls | grep ... | awk ... | xargs img rm

but it would be nice to support clean up of build cache with an option.